The invention relates to a milling tool for machining workpieces of wood, a derived timber product, a plastic material etc.
Milling tools with exchangeable cutting tips are known which cutting tips are fastened by one or more screws to the tool base body. The tool base body has for each cutting tip an L-shaped cutout forming the cutting tip seat in which the cutting tip is positioned. The cutting tip rests at the base of the cutout and in this manner is radially secured. A screw is then pushed through a bore provided in the cutting tip and is then screwed into a thread of a bore provided in the tool base body whereby the screw head is pressed against the cutting tip and the cutting tip is pressed against the tool base body.
German Patent DE 43 25 999 describes a tool in the form of a milling tool for metal machining. This tool is substantially formed by a support having a receiving element and cutting tips. A plurality of exchangeable cutting tips are received the cutting portion which are then fixed in position by screws. The cutting portion has a cutout for receiving the cutting tips and the cutouts are provided with bores which extend from the abutment surface into the cutting portion. This bore is designed to receive a screw which is then screwed into a thread for securing the cutting tip against the abutment surface. The screw has a guide shaft which penetrates the receiving opening fo this cutting tip and extends to a portion of the bore within the cutting portion whereby this portion, relative to the guide shaft, has a clearance fit. In this arrangement, the cutting tip rests positive-lockingly at the cutting tip seat of the base body and at the head of the screw so that the manufacturing tolerances of the cutting tip seat and of the screw can be compensated only by a deformation of the screw. This is possible only for relatively low rpm or masses, i.e., kinetic energies of the cutting tips, so that such a system can be realized for metal machining tools but not for wood working tools.
For machining wood, derived timber products or plastic materials, cutting tips of hard metal are preferably used. The blanks for the cutting tips are already provided with the receiving bores for the fastening screws. These blanks for the cutting tips are then centered and subsequently machined so that the ready-to-use state results. During the sintering process the volume of the cutting tip is reduced by up to 20%. This leads also to a change of the shape in the area of the receiving bore. In order to compensate these changes, the dimensions of the receiving bores are provided with course tolerances.
As already mentioned above, the cutting tips are fastened with screws to the tool base body. Due to the bore tolerances of the cutting tips as well as the screw tolerances, a relatively large play results. The fixation of the cutting tips is then achieved by the frictional force applied by the screw head. For this purpose, screws having preferably a semi-spherical head and having a thread size M3 to M4 are used. Such screws can be tightened only with a comparatively minimal torque which results in the frictional force produced by the screw head and the cutting tip surface being relatively small. In order to ensure that during operation of the tool the resulting centrifugal forces will not surpass the magnitude of the frictional force, it is thus possible only to use correspondingly limited cutting velocities. When the resulting centrifugal forces surpass the frictional force, this causes the cutting tips to move radially outwardly or they are even radially accelerated until the edge of the receiving bore will impact the screw shaft. The diameter of the circle described by the cutting tip is thus increased in an undefined manner so that for a tool having multiple cutting tips only one of the cutting edges will be effective, respectively, offset will result for complementing cutting tips.
The present invention has the object to provide a milling tool of the aforementioned kind as mentioned in claim 1 in which the cutting tips are securely held, even at high load and high centrifugal forces, at the cutting portion by a simple design of the fastening means.